The overall goal of this project is to purify angiotensin converting enzyme from lung (and other tissues) and study its molecular and kinetic properties. A secondary goal is to synthesize a convenient chromogenic substrate. We have made a chromogenic substrate NO2ZGC(S-NDB)G that works well but it is not as soluble as we would like. Because its solubility is considerably below its Km we cannot realize the full sensitivity of this compound as a substrate and it cannot be used to distinguish different types of inhibitors such as competitive and non-competitive inhibition. The latter consideration is an important defect but the inability to obtain full sensitivity is not too important because even at low concentrations the substrate is very sensitive. Another problem with this substrate is that we find it difficult to synthesize and obtain only very low yields. We have been synthesizing different peptides to improve solubility and also to delineate structural features that make compounds good substrates. Our preliminary work indicates in an N-blocked tripeptide, the nature of the blocking group very greatly affects the activity of a substrate.